I recognized that trucks are running slower when I UPGRADE a dirt track to a small road. Before upgrade 40 km/h, after upgrade 30 km/h. I don’t think that an asphalt road should be slower that a dirt track.
Small road is considered as a ‘city’ road and therefore has an enforced speed limit. Use the cross country roads to connect cities, no matter how they render they will be faster than the corresponding city road.